1. Field of the Invention
The present invention relates to an image forming apparatus including sheet feeding means which feeds a recording material toward a transfer area formed between an image bearing member and transfer means, and reversing means which reverses sides of a recording material subjected to an image transfer and an image fixation on a side thereof, for transfer on the other side, and again conveys it to the aforementioned transfer area, wherein continuous image formation is made on both sides of plural recording materials in the order of arrival to the transfer area.
2. Description of the Related Art
For forming images continuously on both sides of plural recording materials (such operation being hereinafter called dual-side continuous image formation), there has principally employed a method of at first executing an image transfer and a fixation continuously on one sides of the recording materials (this operation being hereinafter called one-side image formation), once stacking the recording materials subjected to the one-side image formation, on stacking means which can stack plural recording materials, and then forming images by executing an image transfer and a fixation continuously on the other sides of the recording materials stacked on the stacking means, simultaneously or substantially simultaneously with the completion of the one-side image formation on all the recording materials or after a predetermined time thereafter (such method being hereinafter called a stacking method).
In such stacking method, however, since there is required a certain space for the stacking means inside or outside the image forming apparatus and also since the recording materials subjected to the one-side image formation are to be once stacked on the stacking means until the one-side image formation is completed for all the recording materials, there inevitably result certain limits in the compactization of the image forming apparatus and in the reduction of the time required for image forming process, so that it has been difficult to the recently desired requirements for a compacter image forming apparatus or a faster image forming process.
Consequently there has recently been proposed, instead of the aforementioned stacking method, a method of executing image formation continuously on the plural recording materials in an order of arrival to a transfer area formed between a photosensitive drum serving as a latent image bearing member and transfer means, regardless whether the recording material is conveyed from sheet feeding means which feeds the recording material toward such transfer area, or conveyed from reversing means which reverses sides of the recording material subjected to an image transfer and an image fixation on a side thereof, for image formation on the other side, and again conveys it to the aforementioned transfer area (such method being hereinafter called through-path method). So, an image forming apparatus employing such through-path method has been commercialized to achieve compactization of the apparatus and a faster image forming process.
As a representative example of an image forming apparatus employing such through-path method, there is recently known an image forming apparatus 100 shown in FIG. 5, which is a schematic cross-sectional view of the image forming apparatus 100.
With respect to the image forming apparatus 100, the schematic configuration thereof will not be explained since such configuration is already known, and there will be given an explanation on an image forming process on both sides of a recording material, employed in the image forming apparatus 100 (such process being hereinafter called dual-side image forming process), and on a dual-side continuous image forming process by the through-path method.
At first there will be explained the dual-side image forming process in the image forming apparatus 100, with reference to FIG. 5.
In the dual-side image forming process in the image forming apparatus 100, at first analog image information of an original M, obtained by a scanning exposure with an original illuminating lamp 101 which is movably supported in a direction perpendicular to and in a horizontal direction of the plane of FIG. 5, is converted into digital image information by a CCD 102 which executes A/D conversion, and is stored in an image memory 103 which is capable of storing plural digital information.
Then a control mechanism 104, provided in the image forming apparatus 100, for controlling the functions of various devices, reads the digital image information as the basis of an image to be formed on a recording material, from the image memory 103 according to a predetermined control sequence, and outputs such information to a laser unit 106, which forms an electrostatic latent image corresponding to such digital image information on an external periphery of a photosensitive drum 105 serving as a latent image bearing member.
Receiving the digital image information from the image memory 103 under the control of the control mechanism 104, the laser unit 106 modulates a laser light La according to the entered digital image information, thereby irradiating the external periphery of the photosensitive drum 105 through certain changes in the laser light path.
On the other hand, prior to the laser irradiation by the laser unit 106, the external periphery of the photosensitive drum 105 is given a uniform potential distribution by a primary charger 107, and the irradiation of the laser light La by the laser unit 106 forms an electrostatic latent image corresponding to the given digital image information on the external periphery.
Then, the electrostatic latent image formed on the external periphery of the photosensitive drum 105 is subjected at a predetermined timing to a deposition of a developer from a developing apparatus 108 serving as developing means, and is thus developed into a visible image corresponding to the given digital image information.
Then, the visible image formed on the external periphery of the photosensitive drum 105 is subjected for example to a corona discharge from a transfer charger 109 serving as transfer means, and is transferred onto a side of a recording material which is conveyed at a predetermined timing from sheet cassettes 110, 111 constituting sheet supply means or from a manual insertion unit 112 to a transfer zone Z formed between the photosensitive drum 105 and the transfer charger 109, thus being recorded as an unfixed image corresponding to the given digital image information.
Then, the recording material bearing the aforementioned unfixed image on a side is conveyed, by a conveyor belt 113 provided in the image forming apparatus 100, from the transfer zone Z to a fixing nip N formed between two rollers supported rotatably and in capable of being mutual pressed contact in a fixing apparatus 114 for executing an image fixation by heat and pressure, namely between a cylindrical fixing roller 115 constituting fixing means and a circular rod-shaped pressure roller 116 constituting pressurizing means.
On the recording material arriving at the fixing nip N, the unfixed image is softened, fused and thus fixed by a heat supply from a heating member (not shown) provided as a heat source in a hollow portion of the fixing roller 115 and by a pressure from the pressure roller 116, with a color mixing into a desired color in case of a color image formation. Then the recording material is conveyed through a branching path 117 extending in the image forming apparatus 100 to a dual-side switchback mechanism 118 constituting reversing means which reverses the sides of the recording material, having the transferred and fixed image on a side, for image formation on the other side.
The recording material conveyed to the switchback mechanism 118 is subjected to a reversal of the side already bearing the transferred and fixed image and the other side, then is reconveyed to the transfer zone Z through a re-conveying path 119 extending in the image forming apparatus 100, further subjected to an aforementioned process from the image transfer to the fixation, and is discharged onto a discharge tray 120 supported on a side of the image forming apparatus 100, whereby a dual-side image forming process is completed.
In the following there will be explained, with reference to FIG. 6, a dual-side continuous image forming process by the through-path method in the aforementioned image forming apparatus 100, in an example of using 300 recording materials. The process from the image transfer on a side of each recording material to the fixation on the other side is same as the dual-side image forming process explained in the foregoing, and will not therefore be explained further.
In the dual-side continuous image forming process of the through-path method in the image forming apparatus 100, a unit cycle indicates a time required for image transfer and fixation on one sides of 5 recording materials, which are conveyed from the sheet cassette 110 or 111 (cf. FIG. 5) to the transfer zone in continuous manner with a predetermined interval corresponding to a time of 2 seconds. Such unit cycle is naturally determined by various parameters such as a length of a dual-side path, a longitudinal direction of the recording material, a process speed, etc.
At first, in a first cycle, images are formed by image transfer and fixation in succession on one sides of five recording materials a, b, c, d and e which are conveyed in continuous manner from the sheet cassette 110 or 111 to the transfer zone Z.
In a next second cycle, there are executed alternately (1) formations of unfixed images by image transfers on the other sides of the recording materials a, b, c, d and e which have been subjected to the image formation on one sides thereof and then conveyed to the transfer zone Z, and (2) formations of unfixed images by image transfers on one sides of five recording materials f, g, h, i and j which are conveyed newly in continuous manner from the sheet cassette 110 or 111 to the transfer zone Z.
Thus, in the second cycle, at first the recording material a, already bearing the transferred and fixed image on a side, is reversed in the switchback mechanism 118, then is reconveyed through the reconveying path 119 to the transfer zone Z, then is subjected to an image transfer and a fixation on the other side of the recording material and is discharged (or ejected) onto the discharge tray 120.
Then, after a predetermined time from the conveying of the recording material a to the transfer zone Z, for example after 1 second, a recording material f fed from the sheet cassette 110 or 111 is conveyed to the transfer zone Z, and, after an image transfer and a fixation on a side thereof, is conveyed through the branching path 117 to the switchback mechanism 118.
Then, after 1 second from the conveying of the recording material f to the transfer zone Z, the recording material b, already bearing the transferred and fixed image on a side and reversed in the switchback mechanism 118, is reconveyed through the reconveying path 119 to the transfer zone Z, then is subjected to an image transfer and a fixation on the other side thereof and is discharged onto the discharge tray 120. Thereafter the image transfer and fixation are executed in succession on a side of the recording material g, on the other side of the recording material c, on a side of the recording material h, on the other side of the recording material d, on a side of the recording material i, on the other side of the recording material e and on a side of the recording material j at a time interval of 1 second, whereby the second cycle is completed.
Thereafter, in 3rd to 58th cycles, a process similar to the second cycle is repeated, and, in a last 59th cycle, the five recording materials, each already bearing the transferred and fixed image on a side and reversed in the switchback mechanism 118, are reconveyed through the reconveying path 119 to the transfer zone Z, then are subjected to the image transfer and fixation on the other side of the recording materials and are discharged onto the discharge tray 120, whereby the dual-side image forming process on 300 recording materials is completed.
However, in an image forming method in which (1) recording materials being in an image forming process on a first side and (2) recording materials having the image already formed on the first side and being in an image forming process on a second side are processed in succession or in a random manner, as in the dual-side continuous image forming process of the aforementioned through-path method, the image fixed on the first side and the image fixed on the second side will be different in the surface luster or gloss in case the unfixed images formed on the sides of the recording material are fixed under a same fixing condition.
Such phenomenon may be ascribable to following facts.
An unfixed toner image on a recording material is fixed thereto by heat and pressure in the fixing step. For this reason, a moisture contained in the recording material evaporates by the heat. As a result, in a dual-side image formation on the recording material, the heat capacity of the recording material becomes different in the fixing step for the first side and that for the second side, because of a difference in the moisture contained in the recording material.
Also the recording material holds heat in the fixing step for the first side. Therefore, the temperature of the recording material in the fixing step is different for the fixing step for the first side and that for the second side. Stated differently, if the fixing condition is same for the first side and the second side, a larger amount of heat is given to the toner in the fixing step for the second side. Besides, in the dual-side continuous image forming process of the above-described through-path method, the recording material after the fixing step for the first side is subjected, after passing a dual-side conveying path, to the fixing step for the second side, so that the recording material has a higher temperature at the fixation of the unfixed image on the second side, and the temperature difference of the recording material becomes larger between the fixing step for the first side and that for the second side. It is therefore preferable to employ different fixing conditions for the image of the first side and that of the second side.
In the prior method of stacking the recording materials, after the image formation on the first sides, on the stacking means capable of stacking plural recording materials, and executing the image formations on the second sides after the image formation on the first sides of all the recording materials, it is possible to change the fixing condition for the first side and that for the second side, because the fixing operation for the images of the first sides and the fixing operation for the images of the second sides are executed separately.
However, in the dual-side continuous image forming process of the aforementioned through-path method, it is difficult to maintain an appropriate image by changing the fixing condition for the first side and for the second side, because the recording materials in the image forming process for the first side and the recording materials in the image forming process for the second side after the image formation on the first side are continuously processed in succession or in random manner. More specifically, as in such a configuration that the fixing roller comprises only a heat generating member such as a halogen heater, heat is supplied from the interior of the fixing roller to the surface thereof. This results in a following difficulty. In a control of elevating the surface temperature of the fixing roller for the fixation of the first side and reducing the surface temperature of the fixing roller for the fixation of the second side to a temperature lower than that for the fixation of the first side, in case of fixing an image formed on the side after fixing an image on the first side, the surface temperature of the fixing roller is not easily lowered before the fixing operation is executed. Thus, an elevation of the surface temperature of the fixing roller is possible within a short time by a heat supply thereto, but a reduction in the surface temperature has to wait until a predetermined amount of heat is dissipated. It is therefore difficult to arbitrarily control the elevation and reduction of the surface temperature of the fixing roller within a short time, by regulating the current supply to the heat generating member in the fixing roller.